Electrical power is typically produced at a plurality of generating stations and distributed through a plurality of networks (transmission grid). Power produced at a generating station is typically three-phase alternating current (AC) power. Power from generating stations is conducted to transmission substation where it is converted to high voltages to facilitate long-distance transport on the transmission grid. Power added to the grid must be synchronized with the existing phase of the particular transmission line being utilized to conduct the power. When the power reaches a regional distribution area, the high transmission voltage is typically stepped down at a power substation to accommodate the regional distribution grid. A primary or main distribution power line (feeder line) distributes power from a substation to the surrounding area. Feeder lines may be classified as tie feeders (connects two sources), loop feeders (end connected to a source), radial feeders (connects between a source and a load point), and parallel feeders (connect the source and a load or load center and provide the capability of supplying power to the load through one or any number of the parallel feeders).
Occasionally delivery of power to a group of consumers may be suspended. This may occur when consumer demand for power exceeds production capability causing the network operator to apportion power service to certain customers. This is called load shedding. Load shedding is implemented to avoid automatic shutdown of power stations. To keep the power stations running under such conditions, load shedding may be applied to different parts of the distribution network at set times. For example, critical customers such as hospitals, major factories and, typically, government offices—may get continuous power while parts of the network supplying homes and small business offices may only get power for predetermined time intervals.
Load shedding may also occur during restoration after a fault if there is not enough available power for the demand. When a power outage occurs, fault detection, isolation and recovery (FDIR) programs kick in to isolate the fault and then to restore sections of the distribution network through alternate sources. FDIR systems may be incorporated in utility network management system and may include systems that manage identification and restoration of some or substantially all outage events in a utility grid. In the course of the restoration phase, the FDIR programs may have to shed loads by cutting off power to segments of the distribution network, to restore power into de-energized zones. The shedding of the loads is decided on the last/forecasted loads on the feeders (de-energized zone, alternate sources).
There may be other causes of power outages in a network or sections of a network including, a fault at the power station, damage to the transmission line, damage to substations, short circuits or overloading of electricity mains. If a primary feeder fails, or a fault occurs on a primary feeder or distribution transformer, the other transformers start to feed back through the network protector on the faulted circuit. This reverse power causes the network protector to open and disconnect the faulty supply circuit.
The effect of a power failure is particularly severe in situations where there is an impact on public health and safety or an impact to the environment. For example, hospitals, key communication facilities including emergency broadcast; essential government services such as fire, police, and military; fresh water treatment and waste water treatment facilities; schools designated as emergency shelters; banks; large business media; and individual customers with medical needs would be severely affected during a power outage. Many of these types of customers have back-up power sources.
Operators of power distribution systems, such as for example utilities, closely monitor the reliability and availability of their distribution system. They are normally required by the Public Power distribution network operator Commission (PUC) to record all outages and show progress in improving the extent and frequency of outages. During their rate adjustment period, utilities can are often assessed penalties based on their reliability performance. The power distribution industry has developed performance metrics to measure the reliability and availability of their distribution system. These include the System Average Interruption Frequency Index (SAIFI), System Average Interruption Duration Index (SAIDI), and Customer Average Interruption Duration Index (CAIDI). SAIFI is the average number of interruptions that a customer would experience. SAIDI is the average outage duration for each customer served. CAIDI gives the average outage duration that any given customer would experience. CAIDI can also be viewed as the average restoration time. Operators of power distribution networks are closely monitored by Public Power distribution network operator Commissions (PUC) and may be required to record all outages and show progress in improving the extent and frequency of outages.